The present invention relates to a projectile guidance system for receiving signals of reflected laser energy from a target and generating missile command steering signals, and more particularly, to a low noise preamplifier having wide dynamic range for use in such a laser guided system.
Guiding a missile to a target illuminated by laser energy requires the utilization of the reflected laser energy to generate missile commands. Typically, personnel illuminate a target with laser energy and nearby aircraft, carrying guided missiles, scan the terrain and locate the laser illuminated target by detecting the reflected laser energy. A detector in the missile nose cone is directed at the target and the missile is fired at the target when the target is within range of the missile. The missile is guided to the target by error signals generated by a laser signal processor.
Generally, the applicable system receives the laser energy reflected by the target and that laser light is focused onto a small spot on a quadrant detector. The quadrant detector provides an output signal from the quadrant or quadrants upon which the spot of laser radiation falls. A processor coupled to the output terminals of each quadrant determines the location of the radiation spot and an error signal is generated. Appropriate missile steering commands are generated in response to the error signals.
Laser guided weapon systems generally utilize multiple laser detectors which typically detect very short pulses of laser energies at a relatively low repetition rate. The intensity of the receive signal and consequently the peak value of each pulse may very widely over the trajectory of the weapon. For example, the signals are weak in the maximum range case and increase with decreasing range. Additionally, movement of the target causes changes in reflectivity, variations in the propagation path, presence of smoke and other obstructions in the optical path, and similarly factors can produce rapid and large variations in signals from the detector. To obtain reliable control signals for operation of the guidance system it is necessary to maintain amplitude proportionality among the received pulses. Therefore it is necessary to prevent overloading, clipping, or other non-linearities in the signal circuits.
The energy reflected by the target and focused on the detector is a series of extremely narrow pulses the amplitude of which can increase by a factor as great as 10.sup.6 (120 dbv) as the missile moves toward the target. Generally, a quadrant detector and a preamplifier are utilized. The quadrant detector is a silicon photoconductive junction device operated in a reverse bias mode and behaves as a light controlled current source. Assuming proper loading, the detector output is a series of narrow pulses that closely duplicate the input laser pulse shape. A typical detector output pulse is approximately 20 nsec wide at the half current point and the amplitude can vary over a range of 10.sup.6.
The preamplifier requires a wide bandwidth to be able to pass the narrow pulses and is required to have a low noise figure. A typical dynamic range to be achieved is in the order of 120 dbv. The purpose of the preamplifier is to amplify the detector output with as much fidelity as possible to a level large enough to conveniently process the signal. It is highly desirable that the preamplifier be mounted as close to the detector as possible to reduce the effects of stray capacitances and noise pickup. The preamplifier is a transimpedance device in that it converts the detector output current into a voltage. This allows the detector to be loaded by a low impedance without the addition of Johnson noise. The gain of the preamplifier must be high and for good pulse resolution a bandwidth is necessary up to 20 MHz. Such a bandwidth provides near unity pulse response with minimal pulse stretching with a wider bandwidth reducing the system sensitivity by allowing more noise. Damping must be such that little or no over-shoot occurs. Noise must be kept low of the order of 25 namp rms. Input impedance should be less than 400 ohms to obtain adequate response from the detector since the detector rise time is a function of the load resistance, detector capacitance, and any stray capacitance as well as preamplifier bandwidth. The preamplifier must be a.c. coupled to the detector in order to prevent saturation which could occur if a strong background signal such as the sun is in the field of view. A load resistor is required to provide a d.c. path for the detector bias voltage.
The preamplifier should have a large instantaneous dynamic range as well as a total dynamic range of greater than 120 dbv. The instantaneous dynamic range is needed to acquire targets with a strong input at short ranges. The requirement for such a large dynamic range conflicts with the need for high gain. However, this conflict can be resolve by reducing the gain as a function of incoming signal. The gain may be reduce either by switching to a lower gain at a predetermined signal level or by an automatic gain control (AGC) system where gain is reduce as a continuous function of signal level. However, the gain switching approach has a major advantage of being able to achieve extremely low electronic boresight error. A separate preamplifier is used for each detector quadrant so that the gain of each must be close match to the others over the dynamic range. At each gain level, the preamplifier is a linear amplifier and the gain can be adjusted so that all quadrants match over the entire dynamic range.
One of the major disadvantages of the continuous AGC preamplifier is that when the AGC loop is closed in the system, the preamplifier has a nonlinear response and matching the gain of four nonlinear amplifiers over 120 dbv range is required. Channel balance is difficult to achieve in the last 30 dbv of dynamic range when the missile is close to the target and boresight error has the most effect on accuracy of trajectory.
Accordingly, it is desirable to provide a gain controlled preamplifier having the requisite wide dynamic range, low noise characteristics, wide bandwidth, and substantial linearity with controllable gain conditions for maintaining the most accurate target positional sensing.